This invention relates to resonant-tunneling diodes and to harmonic multipliers which generate harmonics of electromagnetic waves to produce output waves at frequencies above those conveniently available from fundamental oscillators.
Harmonic multipliers are often used, for example, in radio astronomy, where heterodyne receivers, at frequencies above 100 GHz, use power from a harmonic multiplier as the local oscillator source. Harmonic multipliers are also a primary source of power for laboratory molecular spectroscopy in the submillimeter wavelength spectrum.
Harmonic multipliers have traditionally used either the voltage-dependent resistance (a varistor) or capacitance (a varactor) associated with either metal-semiconductor (Schottky diodes) or p-n junctions to generate harmonics of fundamental input oscillators. Capacitive nonlinearities have been favored because of the higher available conversion efficiency between the power generated by the pump oscillator and that available at the desired harmonic. It is known [Page, Proc. IRE 46, 1738 (1958)] that a resistive nonlinearity containing no negative differential resistances can produce a conversion efficiency (ratio of power at the harmonic versus power at the fundamental) of at most 1/n.sup.2 ; where n is the harmonic number. On the other hand, varactors are limited by the Manley-Rowe relations [J. M. Manley and H. E. Rowe, Proc. IRE 44, 404(1956)], which in this case allows a maximum conversion efficiency of unity. In practice many factors lower the efficiency, including the inability to properly terminate all harmonics below the desired output frequency, so as to prevent the bulk of the power being delivered at unwanted frequencies. This last fact greatly increases the circuit complexity of high-harmonic multipliers.
Resonant tunneling structures or quantum well resonators have been described in Chang et al. Appl. Phys. Lett. 24, 593 (1974) and Sollner et al. Appl. Phys. Lett. 43, 588 (1983) and Appl. Phys. Lett. 45, 1319 (1984) and more recently in Goodhue et al. Appl. Phys. Lett. 49, 1086 (1986) incorporated herein by reference. Resonant tunneling occurs when two or more semitransparent barriers are placed closely enough together that charge carriers interact coherently with them. For example, in a mesa-isolated quantum well resonator; a thin (5 nm) layer of material such as GaAs is sandwiched between two thin (5 nm) layers of GaAlAs. The GaAlAs layers form the transparent barriers and the GaAs layer constitutes the quantum well. The addition of the aluminum raises the band gap and the conduction band energy of the barriers above that of the GaAs. These barriers therefore act as partially transparent mirrors to electrons. Charge transport occurs primarily by tunneling through the barriers. The two mirrors from the electronic analog of a Fabry-Perot resonator with peaks in the electron transmission (current) occurring as function of the incident electron energy (voltage). The resonant states formed between the barriers greatly increase the tunneling probability of carriers with energy equal to that of the states, resulting in peaks and valleys in the current as the voltage across the structure is increased. Typically the barriers are made from an epitaxially lattice matched material of larger bandgap than the material sandwiched between. Usually the barriers are formed by replacing Ga with Al atoms when the contacts and the well are made of GaAs.